1. Field of the Invention
The present invention relates to surface acoustic wave filters and, more particularly, to a surface acoustic wave filter having a balanced-unbalanced transformation function.
2. Description of the Related Art
Technological advances in the miniaturization and light-weight design of communication apparatuses such as mobile telephones has been dramatic. As frequency of use increases, surface acoustic wave filters, which are relatively easy to miniaturize, are increasingly used in communication apparatuses. To reduce the number of electronic components and miniaturize the electronic components, efforts have been made to develop components which have a plurality of functions.
Surface acoustic wave filters having a balanced-unbalanced transformation (a so-called balun transformation) function for use in RF stages of the mobile telephone have been actively studied, and are beginning to be used particularly in the GSM (global system for mobile communication) field.
Mobile telephones, depending on the system configuration thereof, require a surface acoustic wave filter having a balun transformation function with the impedance of balanced terminals being at least twice as large as the impedance of an unbalanced signal terminal. Surface acoustic wave filters having the balun transformation function satisfying such a requirement are already in use.
As shown in FIG. 9, the impedance of the balanced signal terminals of a surface acoustic wave filter 1000 is at least twice as large as the impedance of the unbalanced signal terminal. The surface acoustic wave filter 1000 is a 3-IDT longitudinally coupled surface acoustic wave filter including, on a piezoelectric substrate 100, a reflector 106, interdigital transducers (hereinafter referred to as IDTs), each including a comb-like electrode having a plurality of electrode fingers, namely, an IDT 101, IDTs 102, an IDT 103, and a reflector 107 in that order in the direction of propagation of a surface acoustic wave. Each IDT electrode includes a base portion and a plurality of parallel band-like electrode fingers extending from one edge of the base portion in a direction perpendicular to the base portion. The IDT 102 in the center of the filter 1000 includes two separate electrodes, namely, IDT electrode 104 and IDT electrode 105, connected in series in the piezoelectric substrate 100. A terminal 110 connected to the IDT electrode 104 and a terminal 111 connected to the IDT electrode 105 serve as balanced signal terminals. The IDT 101 and the IDT 103 are connected in parallel, and are then connected to an unbalanced signal terminal 113. Other terminals 112 of the IDTs 101 and 103 are grounded.
The surface acoustic wave filter 1000 functions as a filter regardless of whether a serial junction 114 connecting the IDT electrode 104 and the IDT electrode 105 in series is grounded or left as a floating electrode isolated from surrounding elements. The serial junction 114 set to be a floating electrode remains at an intermediate potential between the balanced signals occurring at the terminals 110 and 111 during the operation of the surface acoustic wave filter 1000. The serial junction 114 is substantially at a ground potential. The outermost electrodes 115 and 116 of the IDT 102 are electrodes belonging to the serial junction 114. An electrode finger 117 of the IDT 101 adjacent to the IDT 102 is a signal electrode finger remaining connected to the unbalanced signal terminal 113. An electrode finger 118 of the IDT 103 adjacent to the IDT 102 is a ground electrode finger.
The surface acoustic wave filter having the balun transformation function for use in the RF stage of the mobile telephone preferably achieves an insertion loss as small as possible to a signal in the passband thereof. This is because the signal loss in the RF stage requires more power to increase in amplifier gain in a subsequent stage, and leads to a reduction in an S/N ratio (signal to noise ratio) of the RF stage that significantly effects communication quality.
The degree of balance of balanced signals generated in the surface acoustic wave filter having the balun transformation function for use in the RF stage of the mobile telephone is preferably good. The balanced signal, generated in the surface acoustic wave filter having the balun transformation function for use in the RF stage of the mobile telephone, is input to a differential amplifier at a subsequent stage. If the degree of balance of the balanced signal input to the differential amplifier is poor, the differential amplifier has insufficient performance.
The degree of balance of the balanced signals is a measure indicating how accurately the balanced signals have the same amplitude but opposite phases, and is expressed by an amplitude balance and a phase balance. The amplitude balance is an amplitude ratio of the two balanced signal, and is ideally zero [dB]. The phase balance is determined by subtracting 180° from a phase difference between the two balanced signals, and is ideally zero.
A small insertion loss and a high degree of balanced signals are required of the surface acoustic wave filter having the balun transformation function for use in the RF stage of the mobile telephone. The construction of the surface acoustic wave filter 1000 shown in FIG. 9 increases insertion loss in the passband and degrades the degree of balance of the balanced signals. Such construction will be discussed below.
When the unbalanced signal is applied to the unbalanced signal terminal 113, surface acoustic waves are excited inside the IDT 101 and the IDT 103. As a result, standing waves of the surface acoustic waves are generated in the area of the IDT 101, the IDT 102, and the IDT 103 interposed between the reflector 106 and the reflector 107, both reflecting the surface acoustic waves. The IDT electrode 104 and the IDT electrode 105 in the IDT 102 respectively convert the energy of the standing waves of the surface acoustic waves into electrical energy, thereby generating balanced signals. This is the principle of the operation of the surface acoustic wave filter 1000. What excites the surface acoustic waves is not limited to the inside of the IDT 101 and the inside of the IDT 103 when the unbalanced signal is applied to the unbalanced signal terminal 113. A surface acoustic wave is excited between the electrode finger 117 and the electrode finger 115, between which a voltage is applied, at the border between the IDT 101 and the IDT 102. On the other hand, very little (or no) surface acoustic wave is generated between the electrode finger 118 and the electrode finger 116 at the border between the IDT 103 and the IDT 102. In other words, when the unbalanced signal is applied to the unbalanced signal terminal 113, the excitation of the surface acoustic waves takes place in a bilaterally asymmetrical fashion in the surface acoustic wave filter 1000. Here, the symmetry is with respect to the IDT 102 with the IDT 101 on the left-hand side, and the IDT 103 on the right-hand side.
The bilateral asymmetrical excitation of the surface acoustic waves causes the standing waves of the surface acoustic waves to be asymmetrical, leading to an asymmetry in the distribution of currents flowing in the electrode fingers of the surface acoustic wave filter 1000. In other words, the bilaterally asymmetrical excitation of the surface acoustic waves causes the distribution of currents to be localized in a right half or a left half of the surface acoustic wave filter 1000. The localization of currents in the electrode fingers in a particular area increases energy loss in the form of heat through the resistance of the electrode fingers. The insertion loss of the surface acoustic wave filter 1000 is thus increased.
When the serial junction 114 is grounded, the bilateral asymmetry in the standing wave resulting from the bilateral asymmetrical excitation of the surface acoustic waves becomes a dominant factor which degrades the degree of balance in the balanced signals generated in the surface acoustic wave filter 1000. When the serial junction 114 is grounded, electromotive forces in the IDT electrode 104 and the IDT electrode 105 produce the voltages at the balanced signal terminals. The degree of balance of the electromotive forces of the IDT electrode 104 and the IDT electrode 105 is a dominant factor which determines the degree of balance of the balanced signals. The bilaterally asymmetrical standing waves directly deteriorate the degree of balance of the electromotive forces of the IDT electrode 104 and the IDT electrode 105. Thus, the degree of balance of the balanced signals of the surface acoustic wave filter 1000 is degraded.
The bilateral asymmetry of the excitation of the surface acoustic waves is caused because the surface acoustic waves are generated at the border between the IDT 101 and the IDT 102 while the surface acoustic waves are not generated at the border between the IDT 103 and the IDT 102. Such bilateral asymmetry increases the insertion loss of the surface acoustic wave filter 1000. The degree of balance of the balanced signals is degraded when the serial junction 114 is grounded.
Achieving perfect symmetry in the distribution of the surface acoustic wave excitation is impossible in principle in the structure of the 3-IDT longitudinally coupled resonator type filter in which right and left IDTs are connected in parallel to pick up an unbalanced signal and balanced signals are picked up from the center IDT having two separate electrodes arranged in the direction of propagation of the surface acoustic wave.